So-called "dispersion-shifted" monomode optical fibers are such that at the transmission wavelength at which they are used, which is generally other than 1.3 .mu.m (the wavelength at which the dispersion of silica is substantially zero), the chromatic dispersion of the transmitted wave is substantially zero, i.e. the non-zero chromatic dispersion of the silica is compensated (hence the use of the term "offset") in particularly by an increase in the index difference .DELTA.n between the core and the optical cladding.
The transmission wavelength presently selected for line fibers, i.e. fibers designed to perform long distance transmission, e.g. for transoceanic connections, is substantially equal to 1.55 .mu.m. It is at this wavelength that it is possible to obtain minimum transmission attenuation of light, on the order of 0.2 dB/km.
Thus, in the context of the present invention, the fibers under consideration are designed to be used at a wavelength of 1.55 .mu.m since that is the most efficient for transmission.
Also, it is well known that the bandwidth of monomode optical fibers is much greater than that of multimode fibers, which is why present and future developments of lines for long distance transmission concentrate on monomode optical fibers.
Consequently, the present invention applies most particularly to dispersion-shifted monomode optical fibers designed to be used at a wavelength substantially equal to 1.55 .mu.m.
More precisely, the invention relates to such optical fibers in which curvature losses do not exceed 0.005 dB/m when the radius of curvature is 30 mm. It is well known that such a limitation on curvature losses is necessary to ensure that the optical fiber operates under proper transmission conditions.
At present, numerous dispersion-shifted monomode optical fiber profiles are being studied and they are widely described in the literature.
The simplest known profiles referred to as "step", "trapezium", or "triangle", are such that the refractive index in the core varies as a function of distance from the axis of the fiber, so that when shown as a function of said distance the index appears as a curve constituting respectively a rectangle, a trapezium, or a triangle, while the index in the optical cladding surrounding the core is constant and less than that of the core.
A "pedestal" profile is also known in which the central portion forming the "inner" core of the optical fiber is surrounded successively by an "outer" core of refractive index lower than that of the inner core, and then by optical cladding of index lower than that of the outer core.
Also known is a profile referred to as being of the "trapezium and central ring" type which is shown very diagrammatically in FIG. 1, where there can be seen the curve representing the refractive index n in the fiber as a function of distance d from the axis of the fiber. In that profile, the core C comprises:
a central portion 10 having a maximum index n.sub.s +.DELTA.n in which the index varies in such a manner as to give the curve the form of a trapezium, and in the limit, of a triangle or of a rectangle;
a layer 11 of index n.sub.s, e.g. constant and less than n.sub.s +.DELTA.n, surrounding the central portion 10; and
a layer 12 surrounding the layer 11 and of index n.sub.s +h.DELTA.n (0&lt;h&lt;1), which is constant for example, greater than n.sub.s, and less than n.sub.s +.DELTA.n.
The layer 12 is surrounded by a cladding layer G of index equal to n.sub.s.
In practice, the term "trapezium" when used for the central portion 10 of the core C covers the limiting shapes of a triangle and of a rectangle.
Finally, as described in an article entitled "Transmission characteristics of a coaxial optical fiber line", published in Journal of Lightwave Technology, Vol. 11, No. 11, November 1993, a profile is known of the "buried central hollow" type which is shown very diagrammatically in FIG. 2, where there can be seen the curve of refractive index n in the optical fiber as a function of distance d from the axis of the fiber. In that profile, the core C comprises a central portion 20 of minimum index n.sub.s +h.DELTA.n(h&lt;0) surrounded by a layer 21 of index n.sub.s +.DELTA.n greater than n.sub.s +h.DELTA.n. The layer 21 is surrounded by a cladding layer G' of index equal to n.sub.s.
It is recalled that all of the above-mentioned profiles are naturally circularly symmetrical about the axis of the optical fiber.
All of those profiles make it possible to obtain substantially zero chromatic dispersion at 1.55 .mu.m, while also obtaining low attenuation and curvature losses. Nevertheless, a constant concern in the context of developing long distance links using optical fibers is that of further improving transmission quality and of reducing the cost thereof.
Transmission quality is associated with the signal-to-noise ratio along the link, with noise coming from amplified spontaneous emission generated by the amplifiers belonging to the repeaters used along the transmission line, and it has been shown that this signal-to-noise ratio is itself inversely proportional to a "penalty" function F of the fiber which depends on the distance Z between amplifiers, on the effective mode surface area S.sub.eff of the optical fiber used, on the population inversion factor n.sub.sp, on the linear attenuation .alpha., and on the coupling coefficients C.sub.1 and C.sub.2 respectively at the inlet and at the outlet of an amplifier. The penalty function F is thus given by the formula: ##EQU1##
From that formula, it will be understood that to improve transmission quality, attempts can be made:
to reduce the population inversion factor n.sub.sp, leaving other things equal; nevertheless that require complex development with respect to pumping wavelength and thus concerning line components other than the optical fiber;
to reduce the attenuation .alpha.; nevertheless since attenuation is already very low at 1.55 .mu.m (in practice around 0.2 dB/km), any reduction that can be hoped for will have little influence on the penalty function F;
to act on the coupling coefficients C.sub.1 and C.sub.2 ; that also requires action to be taken on line components other than the optical fiber and therefore requires complex development; and/or
to increase the effective mode surface area S.sub.eff ; that does indeed make it possible to improve the quality of the link.
FIG. 3 shows the penalty function F in dB for an optical fiber using soliton type transmission and plotted as a function of the distance Z in km between amplifiers for a known optical fiber having an effective mode surface area of 50 .mu.m.sup.2 (curve 30) and for a desirable optical fiber having an effective mode surface area of 70 .mu.m.sup.2 (curve 31), with all the other parameters on which F depends being given and remaining unchanged. It can be seen that for given penalty function, i.e. given signal-to-noise ratio, the greater the effective mode surface area, the greater the distance between amplifiers, thus making it possible to reduce the number of repeaters used, and hence reduce the cost of the system.
Also, it can be seen that for given distance between amplifiers, the greater the effective mode surface area, the smaller the penalty function, i.e. the better the quality of transmission.
Hence, to improve transmission quality, or indeed in equivalent manner, to reduce the number of repeaters used for given quality of the link, thus making it possible to reduce the cost of the link, it is advantageous to increase the effective mode surface area.
With simpler index profiles, such as the step, trapezium, or triangle profiles, in order to obtain substantially zero chromatic dispersion at 1.55 .mu.m, i.e. to compensate for the chromatic dispersion of silica at 1.55 .mu.m, it is necessary to increase the index difference between the core and the cladding, thereby necessarily giving rise to a decrease in the effective mode surface area.
Thus, to obtain large effective mode surface areas while ensuring substantially zero chromatic dispersion at 1.55 .mu.m, it is necessary to opt for more complex index profiles such as the profiles shown in FIGS. 1 and 2.
Until now, studies performed on the trapezium and central ring type profile have led to effective mode surface areas that do not exceed 50 .mu.m.sup.2 to 60 .mu.m.sup.2. So far no study has been performed on the buried central hollow profile enabling an effective mode surface area to be determined.